Blender 2.8: Meshes disappear when hitting number keys above the letters
Welcome 

You have 3 Collections (of objects) in the Outliner (see screen shot above).

When you hit the 1 key you are showing only the objects in first Collection.
When you hit the 2 key you see only objects in the second collection etc .

If you look in the Outliner when hitting those keys you will notice how the little Eye icons are being enabled or disabled.

To see all the objects in the 3 Collections at the same time in Object mode again,
you need to go to the Outliner and manually enable the relevant little Eye icons.